Bit Trip

Bash is known for admin utilities and text manipulation tools, but the venerable command shell included with most Linux systems also has some powerful commands for manipulating binary data.

One of the most versatile scripting environments available on Linux is the Bash shell. The core functionality of Bash includes many mechanisms for tasks such as string processing, mathematical computation, data I/O, and process management. When you couple Bash with the countless command-line utilities available for everything from image processing to virtual machine (VM) management, you have a very powerful scripting platform.

One thing that Bash is not generally known for is its ability to process data at the bit level; however, the Bash shell contains several powerful commands that allow you to manipulate and edit binary data. This article describes some of these binary commands and shows them at work in some practical situations.

Viewing and Converting Data

Two tools you can use to represent data in hexadecimal and binary format are hexdump and xxd. The hexdump utility provides many options for outputting hexadecimal data. Although xxd lacks some of the options available with hexdump, it does have one key feature hexdump lacks: In addition to letting you output hexadecimal values, xxd also has the ability to convert a hexadecimal string into binary data. The following command:

command> echo -n "hello" | xxd -p
output> 68656c6c6f

outputs the binary values of the ASCII string "hello" as an ASCII string of hexadecimal values (refer to Table 1).

Table 1

ASCII Lowercase Alphabet

Char

Decimal

Hexadecimal

a

97

61

b

98

62

c

99

63

d

100

64

e

101

65

f

102

66

g

103

67

h

104

68

i

105

69

j

106

6A

k

107

6B

l

108

6C

m

109

6D

n

110

6E

o

111

6F

p

112

70

q

113

71

r

114

72

s

115

73

t

116

74

u

117

75

v

118

76

w

119

77

x

120

78

y

121

79

z

122

7A

In the following command:

command> echo -n "6162" | xxd -p -r
output> ab

The hexadecimal string "6162" is converted into what the xxd man page calls a "mail-safe ASCII representation" of the binary data. Because hexadecimal 61 and 62 correspond to ASCII characters "a" and "b," respectively, the binary data is expressed in the form of the string "ab."

Bash makes it easy to build these commands into functions. The following functions, pack and unpack (named because of their similarity to the PHP pack function) use the preceding commands to convert a hexadecimal string to binary and convert binary to hex.

All the examples so far have used the -p switch to specify that the command will use plain hexdump style, representing each 8-bit byte as a two-digit hexadecimal number. To output data as a binary string (containing 1s and 0s), you need to use different switches. The following function returns a binary string representation of binary data, along with a sample command and output:

The two-character string "ab" is converted into a binary string containing two 8-bit values: 01100001 and 01100010 (97 and 98), corresponding to the base 10 values for characters "a" and "b."

Transforming Binary Data

Bash also offers some commands for transforming binary data using mathematical and logical operations. This process is call arithmetic expansion. In the following example:

command> echo $(( 4*5 ))
output> 20

the output of the echo command is the value calculated by the statement enclosed in $(( and )).

Another tool for performing mathematical operations is bc. bc is a command-line calculator that can read statements from standard input. For example:

command> echo "4*5" | bc
output> 20

Note that the preceding operation is performed in base 10. bc does not provide native functionality for arithematic operations on hexadecimal numbers, so you need to convert the numbers to base 10 and then perform the operation. For instance, if you want to perform simple addition and subtraction operations against hexadecimal numbers, use a function like the following:

The hexadd function provides the ability to add or subtract (add a negative number) to a valid hexadecimal number in a single command. The function pipes a string of commands into bc. The first command, obase=16, sets the base in which data will be output (in this case, hexadecimal or base 16). The second command, ibase=A, sets the base used to read input data. The input base is set up A, which corresponds to base 10. The third and final command is an addition statement comprised of an arithmetic expansion and the second function argument. The arithmetic expansion uses the # operator to convert the number following it from the base specified before the # to base 10. In the command, the hexadecimal value for "A" is converted to decimal 10 using arithmetic expansion, added to 2 using bc, and then converted back into hexadecimal using bc.

You can also perform bitwise operations on the data, including AND, OR, XOR, and shift. The AND operation (using the & operator) returns the bits shared between the two numbers. The OR operation (using the | operator) returns all bits used by either of the two numbers. The XOR operation (using the ^ operator) returns all bits that are unique to one of the numbers.

In Listing 1, the first command performs a binary AND on 3 (or 0011 in binary) and 6 (or 0110 in binary). Because the only shared bit is the second bit, 2 (or 0010 in binary), the output for the command is 2. The second command performs an OR operation against the same two numbers: 3 and 6. Between the two numbers, the lower three bits are used, so 7 (or 0111 in binary) is returned. In the third example an XOR is performed against the same two numbers. The second and fourth bits are the same between the two numbers, however, the third and first bit are different, therefore 5 (or 0101 in binary), is returned.

Listing 1

Bitwise Operations

The other bitwise operations is the shift operation, which shifts the bits in one direction or another (right or left). Listing 2 shows functions for shifting the bits shifting right (using the >> operator) and left (using the << operator).

Listing 2

Shifting Bits

The first command in Listing 2 shifts the bits in 4 (0100) left by two spaces, returning a value of 16 (10000). The second command shifts the bits in 16 (10000) to the right four spaces, returning a value of 1 (0001). As you might have noticed, for each bit space shifted to the left, the number is multiplied by 2, and for each bit space shifted to the right, the number is divided by 2.

Examples

Switching from binary to hexadecimal, and moving bits around to change an A into a C, is certainly interesting, but does this capability have any uses in the real world? The following examples offer some hints for how you could use these tools in practical ways.

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